Knowledge of the enemy's disposition can only be obtained from other men. Hence the use of spies, of whom there are five classes: Local, Inward, Converted, Doomed, Surviving.
When these five kinds of spy are all at work, none can discover the secret system. This is called "diverse manipulation of the threads." It is the sovereign's most precious faculty.
Local spies means employing the services of the inhabitants of a district.
Inward spies is making use of officials of the enemy.
Converted spies, getting hold of the enemy's spies and using them for our own purposes.
Doomed spies, doing certain things openly for purposes of deception, and allowing our own spies to know of them and report them to the enemy.
Surviving spies, those who bring back the news from the enemy's camp.
Hence it is that with none in the whole army are more intimate relations to be maintained than with spies. None should be more liberally rewarded.
Sun Tzu, in Roots of Strategy, Art of War; 500 B.C.